The present invention relates to a pressure sensor for measuring an internal pressure of the fuel tank, capable of detecting a fluctuation of pressure in the fuel system of an automobile engine.
FIG. 13 is a perspective view showing a conventional fuel tank as described in Unexamined Japanese Patent Publication (Kokai) Hei-4-325316. In the drawing, numeral 1 is a fuel tank, and numeral 1a is a fuel tank body. Numeral 2 is a plate to which a pipe 2a, fuel pump 3, fuel level measuring unit 4 and pressure sensor 5 are assembled. Numeral 6 is a rubber packing. FIG. 14 is an enlarged perspective view showing the plate 2 arranged on the fuel tank body and its periphery. Further, FIG. 15 is a cross-sectional view showing the inside of the pressure sensor 5 disclosed, for example, in Postexamined Japanese Utility Model Publication (Kokoku) Hei-6-28664.
In FIG. 15, numeral 7 is a casing of the pressure sensor 5 composed of a casing body 8 and cover 9. Numeral 10 is an atmosphere introducing hole for introducing the atmospheric pressure, which is a reference pressure, provided on a mount 11 of the casing body 8. Numeral 12 is a filter made of a polytetrafluoroethylene porous film provided in such a manner that the filter 12 covers the atmosphere introducing hole 10. This filter 12 is fixed to the mount 11 through a seal member 13 by means of thermal hardening. Numeral 14 is a nipple. Numeral 15 is a semiconductor sensor element to detect a difference between the atmospheric pressure introduced from the atmosphere introducing hole 10 and the pressure to be measured introduced from the nipple 14, and the detected pressure difference is converted into an electric signal by the semiconductor sensor element. Numeral 15a is a diaphragm displaced in accordance with the pressure difference. A gauge resistance that has spread on the diaphragm is changed in accordance with an amount of the displacement. Numeral 16 is a printed circuit board by which an electric signal outputted from the semiconductor sensor element 15 is amplified and corrected so as to obtain a predetermined output.
The fuel tank internal pressure sensor composed in the above manner is used for measuring an internal pressure of the fuel tank 1 and that of the evaporative emission control device, which is not shown in the drawing, connected to the fuel tank.
In this case, commonly, the internal pressure of the fuel tank is very low. In order to measure the low pressure, the sensitivity with respect to pressure is increased by extending the size of the diaphragm 15a in the sensor element 15 and reducing the thickness of the diaphragm 15a, so that an intensity of the obtained signal is increased.
Since the conventional fuel tank internal pressure measuring device is composed as described above, there is a possibility that a liquid containing water gets into the diaphragm portion and the diaphragm may be broken in the case of freezing or by a shock given in the case of collision of a vehicle. When the diaphragm is broken and the vehicle overturns sideways, fuel leaks out.